Over the last decades, the use of digital communication has increased drastically, and a great many digital communication systems have been developed and deployed. For example, the first generation analogue cellular communication systems have now been replaced by 2nd generation digital cellular communication system, such as the Global System for Mobile communication (GSM).
Currently, 3rd Generation cellular communication systems, such as the Universal Mobile Telecommunication System (UMTS), is being deployed. In UMTS, the available frequency resource must be shared between a high number of user equipment and services. In UMTS, the available frequency spectrum is divided into one or few wild band channels having a bandwidth of 5 MHz. Typically, one wide band frequency channel is used for uplink in all cells and a different wide band frequency channel is used for downlink. In this case, separation between cells is achieved through the use of spread spectrum techniques, where each cell is allocated a cell specific long user spreading code.
In these systems, a signal to be transmitted is multiplied by the spreading code, which has a chip rate typically much larger than the data rate of the signal. Consequently, a narrowband signal is spread over the wideband frequency channel. In the receiver, the received signal is multiplied by the same spreading code thereby causing the original narrowband signal to be regenerated. However, signals from other cells having different spreading codes are not despread by the multiplication in the receiver, and remain wideband signals. The majority of the interference from these signals can consequently be removed by filtering of the despread narrowband signal, which can then be received.
Separation between mobile stations of the same cell is also achieved by use of spread spectrum techniques. The signal to be transmitted is multiplied by a user specific code. Similarly, the receiver multiplies the received signal with the user specific code, thereby recovering the originally transmitted signal without despreading signals from any of the other mobile stations. Thus, the interference from all other mobile stations, whether in the same or a different cell, can effectively be reduced by filtering.
A consequence of the spread spectrum techniques employed is that the amount of the interfering signals, which fall within the bandwidth of the narrowband signal, cannot be removed by filtering, and will thus reduce the signal to interference ratio of the received signal. Consequently, it is of the outmost importance that the interference between mobile stations is optimised in order to maximise the capacity of the system. The reduction of the interference from an unwanted mobile station is equal to the ratio between the bandwidth of the spread signal and the narrowband despread signal, equivalent to the ratio between the chip rate and the symbol rate of the transmitted signal. This ratio is known as the processing gain. The technique is known as Code Division Multiple Access (CDMA), and further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.
In order to communicate efficiently in the presence of noise and interference, UMTS as most other digital communication systems provide for error correcting coding and decoding. Hence, the information symbols to be transmitted are encoded by an error correcting code in the transmitter, and the received signal is decoded in the receiver using a suitable decoder. Many different error correcting codes and decoding techniques are known including for example block codes and convolutional codes. Especially, the latter are used extensively in cellular communication systems because they have a high performance and provide a high degree of robustness in the adverse radio conditions frequently encountered in cellular communication systems. Examples of decoder techniques for decoding convolutional codes include Viterbi decoding and Turbo decoding.
UMTS has been developed with a view to provide high data rates and a high number of different services. In order to efficiently communicate high data rates, release 5 of the UMTS Technical Specifications, in contrast to 2nd Generation systems, allows for the use of higher order symbols of more than four constellation points for some services. As the Euclidean distance between different constellation points for the same symbol energy decreases for an increasing number of constellation points, the uncoded error rate increases for higher order symbols. This effect is preferably compensated by use of improved error correcting coding and therefore the performance of the error coding applied to the information data becomes of increasing importance.
Consequently, UMTS (release 5) prescribes the use of highly efficient codes and specifically the use of Turbo decodes. However, these codes are complex and the complexity is increased due to the higher order symbols. Additionally, the error probability of a given bit is increased due to the uncertainty in the decoding process of the value of other bits comprised in the higher order symbols. Furthermore, although the conventional implementation of turbo codes provides high performance, they do not attain the lower theoretical bound for bit error rates. Also, conventional turbo coders assume that the noise is independent from symbol to symbol, which is not the case for multi-level alphabets.
Additionally, any improvement in the performance of error correcting coding and decoding will not only provide for an improvement in the achieved bit error rate but may also allow for lower transmit powers. For a cellular communication system, this may result in reduced interference and ultimately in an improved capacity of the communication system.
Consequently an improvement in performance and/or reduction in complexity of an error correcting system would be highly advantageous.